Ferrofluid sealed rotary feedthroughs are well known in the art as devices for transmitting motion through the walls of vacuum chambers. A typical feedthrough device is shown in FIGS. 1A and 1B. The device employs a housing 10, bearings 12 and rotatable shaft 14, together with a magnet system 16 that is non-rotatably fixed within the housing 10. The magnet system 16 comprises at least one ring magnet 18 and associated magnetic pole piece components 20. Small annular gaps 22 bounded by the outside of the shaft and the inside of the pole piece components are filled with ferrofluid 24 (a colloidal suspension of ferromagnetic particles in a low vapor pressure fluid), which is held in place by the intense magnetic field generated by the magnet system 16. The ferrofluid permits the shaft 14 to turn freely but serves to block the flow of gas axially along the shaft, thereby allowing a pressure difference to exist between the "atmosphere" and "vacuum" ends 26 and 28, respectively, of the feedthrough 100. The particular design shown in FIGS. 1A and 1B uses four ring magnets 18 and five pole rings 20 to produce eight sealing gaps 22. Other magnetic designs e.g., one ring magnet with two pole rings are well known.
Some of the major disadvantages of the prior art are:
1. Very Precise Mechanical Tolerances are Required. If the magnetic gap region deviates very much from a true circular annulus, the seal will have reduced pressure capacity, and may not support the required pressure differential. Consequently, shaft and pole pieces must be aligned with great precision. This requires close-tolerance manufacture of housing, pole pieces and shafts. In devices of the type illustrated in FIGS. 1A and 1B, the dimensions of housing bore, shaft diameter, shaft bearing journals, pole piece outer diameter, and pole piece inner diameter are all typically controlled to within 0.0005 inch of their nominal values. In addition, the pole piece outer and inner diameters are typically specified to be concentric to within 0.0005 inch. These values represent a practical compromise between performance and manufacturing cost.
2. Static Sealing of Pole Pieces is Required.
Sealing must be accomplished in two places in prior art rotary feedthroughs. The first (and obvious) seal is the dynamic seal provided by the ferrofluid between the shaft 14 and pole pieces 20. The second is the static seal provided by O-rings 30 or other materials which seal the spaces between the housing and the outer diameter of the pole piece 20. Elimination of these static seals would simplify the feedthrough, improve reliability and reduce cost.
3. Magnetic Flux Leakage is Significant.
Conventional magnetic fluid feedthroughs 100 (FIGS. 1A and 1B) use main magnetic flux paths that include the magnets, pole rings, fluid gaps and shaft. At the outside diameter of the pole piece, some magnetic flux leaks from one pole ring to the next in a parasitic parallel path. This parasitic external flux represents a minor portion of the total magnetic energy in the system. However, the external flux levels are not zero, and are significant in some applications e.g., electron microscopes.
4. Self-Heating of Ferrofluid at High RPM is Significant. At high RPM, a significant amount of heat is generated in the ferrofluid 24 causing the fluid temperature to increase. Heat must flow through the pole piece 20 to some external heat sink, such as the housing 10 or a water-cooling circuit in the pole piece.